View Full Version : Help! Wood fence installation--How/when to trim posts

Aug. 24, 2011, 10:03 PM
Knee deep in big DIY project--6 acres of 4-board wood fencing. We have the posts in and boards are coming tomorrow. Posts vary in height slightly and will need to be trimmed. Do we trim now at anticipated top height of top rail, or do we wait until top boards are actually up? What tool is most efficient? We have 300+ to trim and they are 5" diameter round pressure treated. Tried circular saw today. Awkward for me! Tried small chain saw. That worked a bit better.

Also, if we wait until boards are up, how do we trim posts flush with top board without knicking/cutting into top board. We want posts level or slightly below level of top board.

Also, with pressure treated lumber, how crucial is it to cut post tops at an angle for rain run-off?


Aug. 25, 2011, 07:56 AM
Trim after boards are up. Trim posts at an angle so water will run off. I think the installers used a chain saw to trim the posts. Good luck with your project :)

Aug. 25, 2011, 08:15 AM
Agree with tasia. With 300 posts, you'll get quite good at it.

Yes, taper top slightly (usually happens anyway with a chain saw). Then BIG SECRET: Seal the top of the post with tar or asphalt paint (the commonly used "fence paint" won't seal it).

Aug. 25, 2011, 08:19 AM
Definately trim after the boards are up. If you want clean cuts use a circular saw ... you can set the angle and the bottom plate of the saw can *travel* on the top board, using the top board to steady the saw.

The problem with using a chain saw is that there is no way to really steady the saw and you will get some *wobble*.

The angle doesn't seem to matter in terms of rain and run-off.

The fence pictured below has a "bull-nose" cut and it was done with a twelve inch circular saw. The measurements were done from the top board (3" high) using a carpenter's square and a pencil (just like building stud-frame construction). All of the posts in the curved sections are 8 X 8s, the straight sections are 6 X 6s.


Aug. 25, 2011, 08:35 AM
If you are making an angle cut where the top of the post is going to be flush with the top board, and then the post top will angle inward and slope down (or the angle will slope outward and down depending on the design of your fence, boards on the inside vs boards on the outside), then figure out how high the plate of the saw needs to rest so that the angled blade will make the cut and NOT knick your top board ... do this on scraps first ... then you know that the saw needs to be maybe 1/2" or 3/4" higher ... now you can mark every post and get accurate, consistent cuts.

Aug. 25, 2011, 08:45 AM
are the posts 4by4 ? ...there are caps made that fit directly onto the posts.... attached with a screw rather than a nail and they will be on forever

Aug. 25, 2011, 03:32 PM
We saved our scraps from the posts. Used the scraps to rest the end of our gates on when they are closed. We nailed the scrap to the post. This takes some of the weight off the gate hardware when the gates are closed.

Aug. 26, 2011, 09:15 AM
Start at the least visible post so that by the time you're at the front you're a pro ;)

Aug. 26, 2011, 10:58 PM
I am guessing that this may be more of a question as to defining the top of the fence over slightly uneven ground: I used a string to align the post holes at grade and then the same stringline raised up to top rail height to "smooth out" the top of the fence line..that way the fence top rail line is straighter and less an exact parallel to every post at a specific height(which could be quite jaggety if the ground is too). This is much harder to explain in writing than to do: I used a measure stick so I could mark say 4 1/2 feet on every post for my top rail height. I would string along on a few nails at the 4 1/2 foot height every 4th or 6th post or so. Once you set the string some posts will measure 4 1/4 feet above grade, others could be 4 3/4 above grade depending on the terrain..just use your judgement, adjusting the stingline with a few nails to average your desired toprail height. Generally I would string about 15 to 20 posts in a run. After setting your toprails you can make up a simple jig to hang all the lower rails for nailing:it will align each rail perfectly parallel to the one above it. Also helps hold the rail at one end while you nail the other. See if you can borrow a Paslode gas nailgun for framing: it shoots a 16d galvanized nail and is totally portable. As Tasia says I would still lop off the tops with a chain saw after mounting the rails.
I put all my posts in, then loaded a trailer with the rails and a generator and a skillsaw and the Paslode. Ran around the fenceline with the truck and trailer, then came back and lopped off the tops.
Believe me when I say that fence building is easy compared to painting the same.

Aug. 27, 2011, 11:36 AM
We never trim fence posts because the pressure treatment does not go all the way through the post and if you trim the top you will remove it and expose the core of the post to the elements. The importance of that probably depends on your location - here in Florida it is critical. Ours were driven in with a hydraulic post driver and any that did not allow the fencing to follow the curve of the land were adjusted either by pulling up slightly or driven the final inches by hand with a large sledge. Hard work, but worth it for beautiful fencing that will last for years. I can't remember off hand the total amount of posts that were put in that way here, but do remember that they came in on three large semis, and they are 6-7 inches wide. Took 2-3 guys several weeks so maybe you just need to give yourself more time. Putting the boards up is the fast part.